1. CMSC201 Computer Science I for Majors Lecture 07 – Strings and Lists
Prof. Katherine Gibson
Based on concepts from:

2. Last Class We Covered
One-way, two-way, and multi-way decision structures
if, if-else, and if-elif-else statements
Control structures (review)
Conditional operators (review)
Boolean data type (review)
Coding algorithms using decision structures

3. Any Questions from Last Time?

4. Today’s Objectives
To discuss the usage of eval() and the potential security concerns
To learn about lists and what they are used for
To better understand the string data type
Learn how they are represented
Learn about and use some of their built-in functions

5. About eval()

6. Previous Uses of eval()
Remember our temperature converter?
What does eval do?
def main():
celsius = eval(input("What is the Celsius temperature? "))
fahrenheit = 9/5 * celsius + 32
print("The temperature is ", fahrenheit,
" degrees Fahrenheit.")
main()

7. The Problem with eval()
eval() interprets a string as code
It lets a Python program run Python code within itself
In our example, we use it to let Python decide what data type to store the input as
If the user gives us an integer, store it as an int
If the user gives us a decimal, store it as a float
Using eval() is a security hole.

8. The Problem with eval()
But if the user gives us a malicious command to delete files or folders, it may also run that
If you have os imported, and you ask for input using eval(input()), someone could type malicious code like in response
os.system('rm hw1.py')
This would delete your hw1.py file!

9. What to Do Instead? Instead of using eval() to cast strings…
Use the exact type you want to cast to:
int(input())
float(input())

10. Fixing the Temperature Converter
Changed to a float cast
def main():
celsius = float(input("What is the Celsius temperature? "))
fahrenheit = 9/5 * celsius + 32
print("The temperature is ", fahrenheit,
" degrees Fahrenheit.")
main()

11. Introduction to Lists

12. Exercise: Average Three Numbers
Read in three numbers and average them
num1 = int(input("Please enter a number: "))
num2 = int(input("Please enter a number: "))
num3 = int(input("Please enter a number: "))
print((num1 + num2 + num3) / 3)
Easy! But what if we want to do 100 numbers? Or 1000 numbers?
Do we want to make 100 or 1000 variables?

13. Using Lists
Need an easy way to hold onto individual data items without needing to make lots of variables
Making num1, num2, …, num99, num100 is time-consuming and impractical
Instead, we can use a list to hold our data
A list is a data structure: something that holds multiple pieces of data in one structure

14. Using Lists: Individual Variables
We need an easy way to refer to each individual variable in our list
Math uses subscripts (x1, x2, x3, etc.)
Instructions use numbers (“Step 1: Combine…”)
Programming languages use a different syntax
x[1], x[0], instructions[1], point[i]

15. Numbering in Lists Lists don’t start counting from 1
They start counting from 0!
Lists with n elements are numbered from 0 to n-1
The list below has 5 elements, and is numbered from 0 to 4 1 2 3 4

16. Properties of a List Heterogeneous (any data type!)
Contiguous (all together in memory)
Ordered (numbered from 0 to n-1)
Have random (instant) access to any element
Add elements using the append method
They’re “mutable sequences of arbitrary objects”

17. List Syntax Use [] to assign initial values (initialization)
myList = [1, 3, 5]
words = ["Hello", "to", "you"]
And to refer to individual elements of a list
>>> print(words[0])
Hello
>>> myList[0] = 2

18. List Example: Grocery List
You are getting ready to head to the grocery store to get some much needed food
In order to organize your trip and to reduce the number of impulse buys, you decide to make a grocery list

19. List Example: Grocery List
Inputs:
3 items for grocery list
Process:
Store grocery list using list data structure
Output:
Grocery list

20. Grocery List Code def main():
print("Welcome to the Grocery Manager 1.0")
// initialize the value and the size of our list
grocery_list = [None]*3
grocery_list[0] = input("Please enter your first item: ")
grocery_list[1] = input("Please enter your second item: ")
grocery_list[2] = input("Please enter your third item: ")
print(grocery_list[0])
print(grocery_list[1])
print(grocery_list[2])
main()

21. Grocery List Demonstration1 2
Here’s a demonstration of what the code is doing
bash-4.1$ python groceries.py
Please enter your first item: milk
Please enter your second item: eggs
Please enter your third item: oil
milk
eggs
oil
milk
eggs
oil
grocery_list[0] = input("Please enter ...: ")
grocery_list[1] = input("Please enter ...: ")
grocery_list[2] = input("Please enter ...: ")
print(grocery_list[0])
print(grocery_list[1])
print(grocery_list[2])

22. List Example: Grocery List
What would make this process easier?
Loops!
Instead of asking for each item individually, we could keep adding items to the list until we wanted to stop (or the list was “full”)
We will learn more about loops in the next couple of classes

23. Strings

24. The String Data Type
Text is represented in programs by the string data type
A string is a sequence of characters enclosed within quotation marks (") or apostrophes (')
Sometimes called double quotes or single quotes
FUN FACT! – The most common use of personal computers is word processing

25. String Examples >>> str1 = "Hello" >>> str2 = 'spam'
>>> print(str1, str2)
Hello spam
>>> type(str1)
<class 'str'>
>>> type(str2)

26. Getting Strings as Input
Using input() automatically gets a string
>>> firstName = input("Please enter your name: ")
Please enter your name: Shakira
>>> print("Hello", firstName)
Hello Shakira
>>> type(firstName)
<class 'str'>
>>> print(firstName, firstName)
Shakira Shakira

27. Accessing Individual Characters
We can access the individual characters in a string through indexing
The characters in a string are numbered starting from the left, beginning with 0
Does that remind you of anything?

28. Syntax of Accessing Characters
The general form is
STRING[EXPR]
Where STRING is the name of the string variable and EXPR determines which character is selected from the string

29. Example String 1 2 3 4 5 6 7 8 H e l o B b
>>> greet = "Hello Bob"
>>> greet[0]
'H'
>>> print(greet[0], greet[2], greet[4])
H l o
>>> x = 8
>>> print(greet[x - 2]) B

30. Example String 1 2 3 4 5 6 7 8 H e l o B b
In a string of n characters, the last character is at position n-1 since we start counting with 0
Index from the right side using negative indexes
>>> greet[-1]
'b'
>>> greet[-3]
'B'

31. Substrings and Slicing

32. Substrings
Indexing only returns a single character from the entire string
We can access a substring using a process called slicing
Substring: a (sub)part of another string
Slicing: we are slicing off a portion of the string

33. Slicing Syntax STRING[START:END] The general form is
START and END must both be integers
The substring begins at index START
The substring ends before index END
The letter at index END is not included

34. Slicing Examples H e l o B b 1 2 3 4 5 6 7 8 >>> greet[0:3]1 2 3 4 5 6 7 8 H e l o B b
>>> greet[0:3]
'Hel'
>>> greet[5:9]
' Bob'
>>> greet[:5]
'Hello'
>>> greet[1:]
'ello Bob'
>>> greet[:]
'Hello Bob'

35. Specifics of Slicing
If START or END are missing, then the start or the end of the string are used instead
The index of END must come after the index of START
What would the substring greet[1:1] be? ''
An empty string!

36. More Slicing Examples H e l o B b 1 2 3 4 5 6 7 8 -9 -8 -7 -6 -5 -4 -31 2 3 4 5 6 7 8 H e l o B b -9 -8 -7 -6 -5 -4 -3 -2 -1
>>> greet[2:-3]
'llo '
>>> greet[-6:-2]
'lo B'
>>> greet[-6:6]
'lo '
>>> greet[-9:8]
'Hello Bo'

37. Forming New Strings - Concatenation
We can put two or more strings together to form a longer string
Concatenation “glues” two strings together
>>> "Peanut Butter" + "Jelly"
'Peanut ButterJelly'
>>> "Peanut Butter" + " & " + "Jelly"
'Peanut Butter & Jelly'

38. Forming New Strings - Repetition
Concatenating the same string together multiple times can be done with repetition
Which operator would you use for this?
>>> animal = "dogs"
>>> animal*3
'dogsdogsdogs'
>>> animal*8
'dogsdogsdogsdogsdogsdogsdogsdogs'

39. Practice: Spam and Eggs
>>> "spam" + "eggs"
'spameggs'
>>> "Spam" + "And" + "Eggs"
'SpamAndEggs'
>>> 3 * "spam"
'spamspamspam'
>>> "spam" * 5
'spamspamspamspamspam'
>>> (3 * "spam") + ("eggs" * 5)
'spamspamspameggseggseggseggseggs'

40. Length of a String To get the length of a string, use len()
>>> title = "CMSC 201"
>>> len(title) 8
>>> len("Help I'm trapped in here!") 24
Why would we need the length of a string?

41. String Operators in Python
Meaning + *
STRING[#]
STRING[#:#]
len(STRING)
Concatenation
Repetition
Indexing
Slicing
Length
for VAR in STRING
Iteration
We’ll cover this next class, when we learn for loops!

42. Just a Bit More on Strings
Python has many, many ways to interact with strings, and we will cover them in detail soon
For now, here are two very useful functions:
s.lower() – copy of s in all lowercase letters
s.upper() – copy of s in all uppercase letters
Why would we need to use these?
Remember, Python is case-sensitive!

43. String Processing Examples

44. Example: Creating Usernames
Our rules for creating a username:
First initial, first 7 characters of last name (lowercase)
# get user’s first and last names
first = input("Please enter your first name: ")
last = input("Please enter your last name: ")
# concatenate first initial with 7 chars of last name
uname = first[0].lower() + last[:7].lower()
print("Your username is: ", uname)
Why is this 7?

45. Example: Creating Usernames
>>> first = input("Please enter your first name: ") Please enter your first name: Donna >>> last = input("Please enter your last name: ") Please enter your last name: Rostenkowski >>> uname = first[0] + last[:7] >>> print("Your username is: ", uname) Your username is DRostenk >>> uname = first[0].lower() + last[:7].lower() Your username is drostenk
Usernames must be lowercase!

46. Example: Creating Usernames
>>> first = input("Please enter your first name: ")
Please enter your first name: Barack
>>> last = input("Please enter your last name: ")
Please enter your last name: Obama
>>> uname = first[0].lower() + last[:7].lower()
>>> print("Your username is: ", uname)
Your username is bobama
What would happen if we did last[7]?
IndexError – but why does last[:7] work?

47. Example: Printing the Months
Given an integer (from 1 to 12) print the three letter abbreviation for that month
Start by storing all the names in one big string:
months = "JanFebMarAprMayJunJulAugSepOctNovDec"
Use the number of the month to get the right “slice” of the months string

48. Example: Printing the Months
Let’s figure out the position for each month name:
months = "JanFebMarAprMayJunJulAugSepOctNovDec"
Month
Jan
Feb
Mar
Apr
May
Jun
Num 1 2 3 4 5 6
Pos 3 6 9 12 15
Month
Jul
Aug
Sep
Oct
Nov
Dec
Num 7 8 9 10 11 12
Pos 18 21 24 27 30 33

49. Example: Printing the Months
Jan
Feb
Mar
Apr
Num 1 2 3 4
Pos 6 9
Notice a pattern?
To get the position, subtract 1 from the month’s number and multiply by 3
pos = (num-1) * 3
Use it to get the month name from the string

50. Example: Printing the Months
def main():
months = "JanFebMarAprMayJunJulAugSepOctNovDec"
n = int(input("Enter a month number (1-12): "))
# compute starting position of month n in months
pos = (n-1) * 3
# grab the appropriate slice from months
monthAbbrev = months[pos:pos+3]
# print the result
print ("The month abbreviation is", monthAbbrev)
main()

51. Example: Printing the Months
bash-4.1$ python months.py Enter a month number (1-12): 1 The month abbreviation is Jan Enter a month number (1-12): 12 The month abbreviation is Dec Enter a month number (1-12): 100 The month abbreviation is
What happened?
months[297:300]
There’s nothing there in the string!

52. Announcements Your Lab 4 is meeting normally this week!
Make sure you attend your correct section
Homework 3 is out
Due by Thursday (Sept 24th) at 8:59:59 PM
Homeworks are on Blackboard
Weekly Agendas are also on Blackboard